Fe(II)-Fe(III) Electron Transfer in a Clay Mineral with Low Fe Content

Iron-containing clay minerals are ubiquitous in soils, sediments, and water and provide a significant source of redox-active Fe that is known to influence metal sorption, contaminant fate, and nutrient cycling. While biological reduction of clay minerals has been known for some time, it has only recently been shown that Fe(II) can abiotically reduce Fe(III) in clay minerals. Here, we used Mossbauer spectroscopy to show that Fe(II) reduces an extensive amount of Fe(III) in a low-Fe clay mineral (Wyoming montmorillonite, SWy-2, 2.3 wt % Fe). The extent of reduction ranges from 12 to 78% over a pH range of 4.0-7.5 and Fe(II) concentration from 0.4 to 2.2 mM and increases as the amount of sorbed Fe(II) increases until about half of the mineral is reduced. It is unclear how such extensive reduction occurs in an Fe-bearing clay mineral with such a low Fe concentration. With only 2.3 wt % Fe in SWy-2, Fe atoms in the clay mineral are spatially isolated and electron conduction or hopping between neighboring Fe atoms seems unlikely and is also supported by Mossbauer spectroscopy collected over a range of temperatures. The lack of evidence for electron hopping in SWy-2 raises the intriguing question of how the electrons access Fe deep in the clay mineral structure. Data from Mossbauer spectroscopy and chemical extraction suggest that electron transfer through the basal plane rather than edge sites may explain how such extensive reduction occurs in SWy-2. Electron transfer through the basal plane would provide access to the Fe atoms throughout the structure without requiring electrons to move in the structure via hopping or conduction. Our work demonstrates that Fe(II) reduction of clay minerals can occur across a range of geochemically relevant conditions and that extensive reduction can occur in low-Fe clay minerals possibly via electron transfer through the basal plane.